Field of the Invention
The present invention relates to an image pickup apparatus, such as a digital camera, that performs contrast detection focus control, a method of controlling the image pickup apparatus, and a storage medium.
Description of the Related Art
When photographing a scene in the open air using a digital camera, there is a case where a luminance range of the scene is wider than a photographable luminance range. In this case, the gradation information of an object outside the photographable luminance range cannot be recorded, and hence blown-out highlights or blocked-up shadows are formed in the photographed image. For example, in the case of photographing a person in the open air on a clear day, if the exposure is adjusted for the person, there can be a case where the photographed image suffers from blown-out highlights in the sky or clouds as the background or blocked-up shadows in the leafy shade of a tree as the background. Therefore, the impression given when the photographed image is viewed is sometimes different from that given when the scene is actually viewed.
As one of techniques for solving the above-mentioned problem, an HDR (High Dynamic Range) process is known. The HDR process basically includes a dynamic range-increasing process and a gradation compression process.
The dynamic range-increasing process increases the photographable dynamic range and records gradation information of a luminance range in which blown-out highlights or blocked-up shadows occurred. According to a general method thereof, a group (multistage exposure image group) of images photographed by changing the exposure for the same contents are synthesized (hereinafter, an image generated by this dynamic range-increasing process is referred to as an HDR image).
On the other hand, the gradation compression process is image processing for reproducing an HDR image having a wide dynamic range using a display/output device with a narrow dynamic range, in a preferable manner, and uses a method of compressing amplitude or the like, for low-frequency components of the luminance of the HDR image. By executing the HDR process described above, it is possible to display or print out a preferable image in which the blown-out highlights/blocked-up shadows are reduced (Japanese Patent Laid-Open Publication No. 2007-215073).
On the other hand, during photographing of a moving image, many of the digital cameras perform AF (auto focus) control by evaluating the contrast of a photographed image. However, in a frame image picked up over a long exposure time period, i.e. at a slow shutter speed, the image can be out-of-focus e.g. due to a camera shake, causing high-frequency components to be lost, and hence it is not possible to always evaluate the contrast with accuracy.
To solve this problem, there has been proposed a technique which sequentially performs image pickup over a short exposure time period and over a long exposure time period in an alternating manner, and uses a frame picked up over the long exposure time period for display and recording, and a frame picked up over the short exposure time period for image evaluation e.g. for AF (Japanese Patent Laid-Open Publication No. 2009-141538).
However, the above-mentioned Japanese Patent Laid-Open Publication No. 2007-215073 and Japanese Patent Laid-Open Publication No. 2009-141538 have no description about image evaluation using images picked up under respective exposure conditions, which images are picked up by changing the exposure condition for image synthesis in moving image photographing for the purpose of generating an HDR moving image having a wide dynamic range.